In a conventional micro fabricated electron emission device, metals, silicon and their compounds as for materials of the device have been used. Such a material on its uppermost surface as the electron emission face becomes easily oxidized in air, forming an oxide in which work function is high and has made the electron emission device with high work function. Also, an oxide such as of metal or silicon by its insulating nature is hard to emit electrons.
On the other hand, if a nanocarbon material such as carbon nanotubes having a size in the order of nanometers (nm) is utilized, there can be no surface oxidation, avoiding a rise in work function. Further, the nanocarbon material has a high conductivity and also has a high thermal conductivity. Therefore, it can readily emit electrons and its failure by Joule heat can be prevented.
By the way, many attempts have been made to use nanocarbon material as the material for electron emission devices. As is well known, however, many conventional nanocarbon materials made by utilizing plasma contain a lot of impurity components such as amorphous substances and there has been no method that can reproducibly make highly aligned carbon nanotubes especially suitable to emit electrons uniformly.
Since carbon nanotubes have an extremely small radius of curvature at their apical ends and are a material extremely stable mechanically and chemically for use in electron emission device, use in an electrode of a double layer capacitor as they have graphite edges at high density and use in an electrode of a fuel cell as they can adsorb and emit ions at high density, investigations have been advanced to put carbon nanotubes to practical use.
By the way, if used in an electron emission device, an double layer capacitance electrode or a fuel cell electrode, carbon nanotubes are used in a bundle in which they are aligned unidirectionally and at high density and namely as high-density and highly oriented carbon nanotubes. It is evident that their performances are enhanced as their density and orientation are increased. To wit, the higher the density and orientation, the higher the electron emission efficiency per unit area for the electron emission device and the larger the output of power storage or generation per unit area for the double layer capacitor or fuel cell electrode. Consequently, densely aligned and highly oriented carbon nanotubes that are much higher in density and orientation are being needed.
Since carbon nanotubes are a nano material having a radius in the order of nanometers, it is impossible to synthesize carbon nanotubes randomly aligned and then to mechanically orient the individual carbon nanotubes and align them densely. Accordingly, in making densely aligned and highly oriented carbon nanotubes, a synthesizing method that carbon nanotubes are automatically aligned onto the substrate has been used. As one of such methods, there is known a method of synthesizing densely aligned and highly oriented carbon nanotubes by heating a substrate carrying a metal catalyst in an organic liquid (See Patent Reference 1.). This method, which is based on a peculiar interfacial cracking reaction that occurs when a solid substrate and an organic liquid contact having a high temperature difference, is called solid-liquid interfacial contact cracking in organic liquid. This method, which is based on a peculiar interfacial cracking reaction that occurs when a solid substrate and an organic liquid contact each other having a high temperature difference, is called solid-liquid interfacial contact cracking in organic liquid. According to this method, the highly pure carbon nanotubes can be synthesized without forming an impurity such as amorphous carbon as in arc discharge and chemical vapor deposition methods (See Non-patent Reference 1.).
The method according to Patent Reference 1 is mentioned referring to FIG. 18. The Figure shows a synthesis apparatus using the solid-liquid interfacial contact cracking method. The synthesis apparatus comprises a liquid tank 31 retaining an organic liquid 30 such as methanol, a water cooling means 32 provided to surround the outside of the liquid tank 31 for maintaining the organic liquid 30 at a temperature under its boiling point, a substrate holder 35 supporting a conductive substrate 33 and having an electrode 34 for flowing an electric current through the substrate 33, a condensing means 37 comprising a water cooling pipe 36 that cools and condenses a vapor of the organic liquid from the liquid tank 31 to return the condensate to the liquid tank 31, a N2 gas inlet valve 38 through which N2 gas is introduced to prevent the organic liquid vapor and air from contacting each other, and a cover 39 for sealing the liquid tank 31.
To perform the solid-liquid interfacial contact cracking in the organic liquid with the synthesis apparatus above, a thin film of metal catalyst of a transition metal such as Fe, Co or Ni is layered on a silicon or diamond substrate and the substrate is exposed to hydrogen plasma to cause the substrate to carry fine particles of the metal catalyst densely distributed thereon. This substrate 33 is held by the substrate holder 35 and an electrical current is flown through the substrate 33 via the substrate holder 35 to heat the substrate 33. This causes carbon nanotubes to be synthesized and built on the fine particles of metal catalyst by a peculiar interfacial cracking reaction brought about when the substrate 33 and the organic liquid 30 come into contact with each other having a severe temperature difference. According to this method, carbon nanotubes which are highly oriented perpendicular to the substrate and densely aligned can be synthesized wherein making the thin film of metal catalyst thinner allows the fine particles of metal catalyst to be finer in particle size and more densely distributed. As shown in FIGS. 19(a) and 19(b), densely aligned and highly oriented carbon nanotubes having a radius of about 20 nm and aligned at a density of 300 tubes/μm2 perpendicular to the substrate can thus be synthesized.    Patent Reference 1: Japanese patent laid open application, JP 2003-12312 A    Patent Reference 2: Japanese patent laid open application, JP 2004-327085 A    Non-patent Reference 1: National Institute of Advanced Industrial Science and Technology, Research Center for Nanocarbon editing: Corporation's Series Nanocarbon Materials “Dream Expanding New Materials” (in Japanese), May 25, 2004, published by Maruzen Co., Ltd. pp. 155-157, p. 198, and Table 2